Rotors for disk brake applications are well known in the art. Braking systems utilizing rotors operate by application of a compressive force upon opposing surfaces of the rotor, typically by brake pads, which results in the generation of friction between the pads and the rotor. This friction acts to slow the rate of rotation of the rotor, and thereby of the wheel to which it is fixed, such as, for example, a wheel of an automobile. In the case of an automobile, application of the compressive force ultimately converts the kinetic energy of the automobile into thermal energy as the automobile is slowed by the friction. It is readily understood that materials used to fabricate rotors must exhibit suitable strength to withstand the compressive forces, frictional forces, and the resulting torque, to which the rotor is exposed, and must have a high capacity to dissipate the thermal energy generated by friction during braking. An additional property that is important in some applications, such as, for example, when the rotor is used in a racecar, is the weight of the rotor. A desirable rotor for racing applications provides suitable slowing ability without adding substantially to the total weight of the vehicle.
While the properties of strength, heat dissipation and weight are all important to brake rotor design, it is appreciated that certain desirable features are often compromised in designs where other properties are improved. One important aspect of rotor design and manufacture that bears upon these properties is the selection of the material from which the rotor is made. For example, materials such as stainless steel have excellent strength characteristics, but are relatively heavy, indeed unacceptably heavy for many applications. The use of aluminum alloy materials enables construction of a lighter weight rotor without compromising thermal conductivity, but the use of aluminum compromises strength compared to steel. Titanium alloys combine high strength and low weight but exhibit lower heat conductivity compared to steel and aluminum.
One process described in the prior art for the manufacture of rotors includes casting of a material in a rough form, followed by finishing of surfaces to provide a finished rotor. In casting processes described in the art, rotors are typically made from ferrous alloys which are cast into a desired shape and machined in multiple operations into a finished rotor. Casting processes have also been described in which rotors arc made using composite materials such as carbon fiber. Casting does make it possible to incorporate internal cooling features such as fins into the rotor's design which improves the rotor's heat dissipation properties and decreases the weight of the rotor; however, cast rotors made from ferrous alloys are relatively heavy, and are therefore not well suited for many applications.
In other fabrication processes described in the art, rotors are fabricated using multiple layers of material, typically metal, which are then fused or otherwise joined together into the final desired structure. The different layers of the rotor may be made from different materials such as steel, titanium and aluminum alloys in an effort to achieve a combination of properties desired. This method allows for rotors having more complex internal cooling features such as those shown in U.S. Pat. No. 5,626,211 to Gewelber et al.; however, it is believed that this method of manufacture has not gained widespread appeal because it involves many steps leading to high production costs and an increased incidence of defects in the rotor.
In another manner of making rotors, rotors are machined from a solid billet of material. Machined rotors known in the art include cooling features cut in the friction surface of the rotor, such as, for example, cross-drilled holes passing through the friction surface of the rotor and surface slots cut in the friction surface. Cross-drilled holes and friction surface slots are two examples of features that decrease the weight of the rotor as well as increase its cooling capacity; however, this approach decreases the available surface area for generating friction between the rotor and brake pads, thereby reducing the overall braking efficiency of the rotor.
There is a continuing need for brake rotors that are lightweight and possess high strength and high heat-dissipation capacity, and methods for making same, particularly for use in many high speed and heavy braking applications. This is particularly true in racing applications such as NASCAR and in sprint racing where a single rotor, the rear left, often bears most of the burden during braking. The present invention addresses this need.